Fungi/pests in your lawn, It appears to much of humidity breeds fungiTry to get it dry, or else, just peel an onion, cut it in thin slices,place it 2/3 inches deep in the soil, one yard away from each other.

I am an Organic Gardener, treating plants with Herbs & SpicesYour problem, it appears lack of iron, just place a rusty nail 2/3 inches deep in the soil, a feet away from the plant keep moist.I feel it should work